1. Field of the Invention
As is well known, nitrogen and phosphorus are two of three chemical elements that are essential macronutrients for living plants. The third essential macronutrient for living plants is potassium. Chemical nitrogen is usually applied to soil in solid forms, such as urea, ammonium nitrate, and ammonium sulfate and in solution forms, such as anhydrous ammonia, ureaammonium nitrate solutions, and various ammoniated or ammonia solutions; however, the most prevalent form of solid nitrogen used for agriculture today is urea.
As is also well known, there are certain drawbacks as well as advantabes for using solid urea for agriculture. Currently, urea affords the highest nitrogen content of all solid commercially available fertilizers. Among the other advantages of urea over that of ammonium nitrate in the agricultural market is that it is a nonhazardous material and can be transported by barge or other mode of transportation and can be handled at any port or distribution facility because it is not sensitive to detonation and does not burn, but decomposes to ammonia and biuret with heat. The relative ease with which urea decomposes by hydrolysis to ammonia and carbon dioxide has been recognized by the agronomist as a serious problem in that nitrogen applied in the form of urea to the soil and especially when applied to the soil surface can be lost by upwards of about 80 percent by decomposition and volatilization to the atmosphere. Ammonium nitrate, the second most highly used solid nitrogen containing commercial fertilizer, can be, under certain circumstances, considered a hazardous material and thereby its movement in the United States is limited as to mode of transport; consequently, rates incurred in its movement are greater than are rates for nonhazardous solid urea.
Agronomists have long recognized the disadvantageous characteristics of solid urea as a nitrogen fertilizer source. In an effort to overcome some of the disadvantages and drawbacks sometimes associated with the use of solid urea as a nitrogen fertilizer source, the research of Dr. Khasawneh at the Tennessee Valley Authority has led to the discovery that urea phosphate, which by itself, is an efficient nitrogen source, possesses the unique property of reducing nitrogen loss from urea with which it may be co-applied to soil surfaces. See Defensive Publication No. T102,902, supra.
2. Description of the Prior Art
In view of the discovery by Dr. Khasawneh, supra, and the envisioned future potential for urea phosphate, a study was made with the objectives in mind of developing a process for the direct granulation of urea phosphate product from urea and merchant-grade wet-process phosphoric acid without first removing the acid impurities. Prior art in the field of manufacture of urea phosphate fertilizers from urea and wet-process orthophosphoric acid of about 30 to 70 percent P.sub.2 O.sub.5 concentration is well documented in the patent literature such as, for example, U.S. Pat. No. 3,713,802, Gittenait, which describes a process for reacting urea and wet-process phosphoric acid to form crystalline urea phosphate and for separating the relatively pure crystalline product from the mother liquor containing the acid impurities, e.g., iron, aluminum, calcium, magnesia, and fluorine. Prior art also in this field is described in other patents such as U.S. Pat. No. 3,936,501, Greidinger et al; Belgian Pat. No. 665,219, W. R. Grace and Co., Oct. 1, 1965; and German Pat. No. 2,100,413, Marcel Gittenait, July 15, 1971; all of which describe processes for producing urea phosphate from urea and phosphoric acid by crystallization and subsequent separation of the crystals from the mother liquor containing the acid impurities. Prior art also is described in a paper, "Production of Granulated Urea Phosphate Fertilizer in a Fluidized Bed," Soviet Chemical Industries 5 (8), August 1973. The process described in the paper involves reacting urea and phosphoric acid in a fluidized bed in the presence of hot (180.degree. to 250.degree. F.) air to cause granulation. The urea melt and the acid are sprayed into the fluidized bed, the urea melt at least partially decomposes to liberate ammonium vapor, which vapor is reacted with and neutralizes said phosphoric acid. Also, the equipment required for effecting the described process is sophisticated and is not at this time conventional in the fertilizer industry. Furthermore, the heat requirement for granulation appears to make the process energy inefficient.